when mine need sharpening, they start leaving streaks in the print after I pull... like little nicks in the blade that just don't pick up any ink.
get some scrap wood, like maybe 2 lengths of 1" x 6" (slightly longer than your longest squeegie). Attach them to each other perpendicularly so that what you have looks like a long, skinny half-box, or trough-looking thing. Glue some fine grit (NOT 80 grit!) sandpaper to the inside surface of one of those boards... the squeegee stays square along the inside surface of the other board, and you can gently work the squeegee back and forth across the sandpaper, giving it a new edge.